1. Field of the Invention
This invention relates to measuring mass air flow using a sensor.
2. Prior Art
A conventional hot wire anemometry based mass air flow sensor (MAFS) has two small temperature dependent resistive elements placed in the air flow stream. One element (called the hot element) is heated, via the use of electrical power dissipation, to a high temperature. The other element (called the cold element) is used to sense ambient air temperature. The circuit used with these elements is normally made with thick film resistors which add flexibility to the manufacturing process. These resistors may be laser trimmed to optimize the sensor function. The current through the elements is monitored through a resistor creating a voltage V.sub.sig. Voltage V.sub.sig provides the input for an output section of a mass air flow sensor assembly circuit shown in FIG. 1. A voltage V.sub.out is at the output of the output section and coupled to output resistors R.sub.1, R.sub.2, R.sub.3, and R.sub.4.
When calibrating a mass air flow sensor it is known to trim the output resistors in order to move the transfer function relating mass air flow output voltage to the air flow being sensed (see FIG. 2). After the output resistors are trimmed, a new output voltage is measured. If the output voltage is still below the desired output voltage (corresponding to the desired transfer function) then the resistor is trimmed again and a measurement of output voltage is made again (see FIG. 3). These voltage measurements are relatively unstable because they are a function of the air flow. That is, it will take approximately about two seconds to get an average value of voltage output which can be deemed highly accurate (see FIG. 4). Because of this two second time delay after each cut to trim the resistor, the process of obtaining a final cut in the resistor trim procedure is undesirably long. It would be desirable to shorten the process of cutting the resistors which form a part of the mass air flow sensor sensing system.
In summary, due to mass air flow sensor assembly part to part variation, mass air flow sensor output stage resistors R.sub.1, R.sub.2, R.sub.3, and R.sub.4 must be laser trimmed to meet sensor transfer function requirements. A typical transfer function calibration process exposes the sensor to a low air flow and trims resistors R.sub.1 and/or R.sub.2 to adjust V.sub.out to a desired value and to provide an offset adjustment. The sensor is exposed to a high air flow while trimming resistors R.sub.3 and/or R.sub.4 to adjust V.sub.out to a high air flow desired transfer function and to provide a gain adjustment. The low air flow V.sub.out is then re-checked to ensure it has not shifted too much during the high air flow adjustment. If the low air flow V.sub.out is unacceptable both process steps must be repeated. This process is time consuming due to two reasons. First, all of the resistor trimming is performed at a time when air flow is passing through the mass air flow meter and creating V.sub.sig. Voltage V.sub.sig , due to the inherent dynamics of air flow, will be noisy. Therefore, to ensure accuracy lengthy measurements of V.sub.out must be performed during the laser trimming. Second, since adjusting the resistors at the high air flow will force the low air flow V.sub.out to shift, the process of going from low to high air flow laser trimming, often must be repeated, further adding to calibration process cycle time. These are some of the problems this invention overcomes.